The development of various devices and systems in recent years has increased the demand for higher performance batteries as a power source (primary battery, secondary battery, capacitor, etc.). For example, lithium secondary batteries are gaining widespread popularity as high energy density secondary batteries serving as the power source for electronic devices, such as portable communication devices, laptop computers, etc. Further, in terms of reducing environmental load, lithium secondary batteries are also expected to be used as batteries for driving the motors for vehicles. Accordingly, there is a demand for the development of high energy density lithium secondary batteries that will correspond to higher performance in the above devices. In order to meet this demand, increasing the capacities of both positive electrodes and negative electrodes is necessary.
However, the capacity of the positive electrode for currently available lithium secondary batteries has not increased as much as that of the negative electrode. For example, the specific capacity of lithium nickel oxide-based materials, which is said to be relatively high, is about 190 to 220 mAh/g. Even in the case of Li2MnO3 based materials, which contain a relatively larger amount of lithium per formula weight, their theoretical capacity based on the assumption that all of the lithium ions are used during charge and discharge, is merely about 460 mAh/g.
In contrast, although sulfur is a substance with a low operating potential, the theoretical capacity thereof is as high as about 1670 mAh/g. However, there are problems such as the fact that elemental sulfur has a low conductivity, and reacts with lithium ions during the discharge of a battery system including currently available organic electrolyte (for example, an electrolyte in which LiPF6 at a concentration of 1 M is dissolved into a 1:1 mixed solution of ethylene carbonate and dimethyl carbonate), and thereby dissolves into the electrolyte. Using metal sulfides (MSx; M represents a metal component such as nickel), which have a conductivity comparable to or higher than that of a semiconductor material and shows relatively low dissolution into the electrolyte compared to sulfur, is one approach to overcoming these problems. Because MSx reacts with two lithium atoms per sulfur atom during charge and discharge, the number of sulfur atoms per formula weight should be increased as much as possible in order to design a high capacity positive electrode. For example, when M represents nickel, the theoretical capacity of NiS, wherein x=1, is about 590 mAh/g, whereas the theoretical capacity of NiS2, wherein x=2, is about 870 mAh/g.
However, because sulfur ignites in air at about 250° C., and its melting point is as low as about 120° C., controlling the composition of a product is difficult.
Accordingly, an extremely complicated process is employed as a process of producing a metal sulfide MS2 with a high proportion of sulfur atoms. For example, a metal and a sulfide are reacted by conducting heat treatment for a long time while controlling sulfur vapor pressure in a reducing atmosphere. In order to complete the synthesis of the metal sulfide in a relatively short period of time, it is necessary to conduct the reaction in a stream of H2S or in a high-pressure gas atmosphere at a high-temperature.
However, the above-described processes require a prolonged heat treatment or a reaction that takes place in a high-pressure gas atmosphere. Further, Synthesis carried out using a stream of H2S requires exhaust gas treatment. Additionally, sulfide obtained by the process that uses a stream of H2S has poor crystalline quality. Accordingly, an easier production process is needed in order to promote the use of metal sulfide as a positive-electrode material for lithium secondary batteries.
Mechanical milling, precipitation reaction in liquid phase, etc., are known to be easier than the above-described processes. Of these, a production process using mechanical milling supplies less energy to a sample powder, as described, for example, in Non-Patent Document 1, compared to the thermal reaction process. However, although NiS, wherein x=1, can be easily produced by mechanical milling, NiS2, wherein x=2, is difficult to produce. Further, as described, for example, in Non-Patent Document 2, although a production process using a precipitation reaction in liquid phase can produce crystalline Ni3S2 (x=2/3), Ni3S4 (x=4/3), etc., by adjusting the pH of a reaction solution, it is also difficult to produce NiS2 (x=2) with this process.
Among the above-described metal sulfides, iron sulfide contains iron element, which is a relatively inexpensive metal, as a raw material. Thus, an iron sulfide with a high proportion of sulfur atoms is considered to be useful as a positive-electrode material that is inexpensive and has a high theoretical capacity.
The following process, for example, is known as an iron sulfide (FeS2) production process that has a high proportion of sulfur: iron and sulfur are mixed in a molar ratio of 1:2, iodine is added thereto, and then the mixture is vacuum-sealed in a quartz tube and thermally reacted for about 5 days (see Non-Patent Document 3 below). The following process is also known: after an NaOH aqueous solution saturated with H2S is added to a mixture of FeSO4·7H2O and rhombic sulfur, H2S is passed therethrough, and the reaction system is closed and kept under heat for about 2 weeks (see Non-Patent Document 4 below).
Further, the following process is also known: iron powder and sulfur powder are mixed in a molar ratio of 1:2, and the mixture is placed in a tungsten carbide container and mixed in a ball mill for about 110 hours under an atmosphere filled with Argon gas, thus producing iron sulfide (see Non-Patent Document 5 below).
However, these processes have disadvantages that precise atmospheric control is required during production and the reaction time takes long time exceeding 100 hours in order to obtain intended iron sulfide.
As described above, an easy production process for a metal sulfide with a high proportion of sulfur atoms has not been established. Accordingly, the development of a technique for easily producing a metal sulfide MSx (x>1) is desired in order to promote the widespread use of a high capacity lithium battery that uses a sulfur-based positive electrode.    Non-Patent Document 1: S-C. Han, H-S. Kim, M-S. Song, P. S. Lee, J-Y. Lee, and H-J. Ahn, J. Alloys Comp., 349, 290-296 (2003).    Non-Patent Document 2: Y. U. Jeong and A. Manthiram, Inorg. Chem., 40, 73-77 (2001).    Non-Patent Document 3: G. Brostingen and A. Kjekshus, Acta Chem. Scand., 23, 2186 (1969)    Non-Patent Document 4: R. A. Berner, Econ. Geol., 64, 383 (1969)    Non-Patent Document 5: J. Z. Jiang, R. K. Larsen, R. Lin, S. Morup, I. Chorkendor., K. Nielsen, K. Hansen, and K. West, J. Solid State Chem., 138, 114 (1998)